Estimation of the ductility and hysteretic energy demands for soil–structure systems

Abstract

This study aims to consider the effect of soil–structure interaction (SSI) on the ductility and hysteretic energy demands of superstructures and propose empirical equations for demand prediction in soil–structure systems. To this end, the FEMA 440 procedure was considered to develop nonlinear single-degree-of-freedom oscillators with a period range of 0.1–3.0 s, as the representative of superstructures. The elastic-perfectly plastic and a moderate pinching degrading hysteretic models were considered for the nonlinear response of the superstructure. The model of the nonlinear soil–foundation system was developed through the Winkler method. In this regard, the type of soil beneath the foundation was assumed as D category, according to the site classification in ASCE 7-10. A wide range of key parameters, including the strength reduction factors (2 ≤ Rμ ≤ 8), the foundation safety factor (3 ≤ SF ≤ 7), the foundation-to-structure height aspect ratio (1 ≤ h/b ≤ 5), and the foundation length-to-width ratio (3 ≤ Lf/Bf ≤ 20) was introduced into the analytical models to conduct parametric studies. Results show the considerable effect of SSI on reducing the ductility and hysteretic energy demands in superstructures with short fundamental periods. More demand reduction can be achieved by providing the lateral sliding of the foundation on the soil surface, especially for systems with a small aspect ratio. The pinching–degrading hysteretic behavior of the superstructure remarkably modifies the level of demands. Moreover, predictive models were proposed for estimating the ductility and hysteretic energy demands in flexible base systems. These models modify demands in the rigid base structures based on their physical and mechanical properties. The developed models consider the effects of structural hysteretic behavior as well as foundation flexibility. The efficiency of the proposed model was assessed on a multi-story frame. Finally, the required ductility capacity of the systems was determined through the Park–Ang damage index and by using the developed predictive models. Results show the efficiency of the empirical models to reasonably estimate the required ductility capacity.